1. Field of the Invention:
The present invention relates to in situ bioremediation of contaminated soil and groundwater. In particular, the invention relates to an apparatus and method for adding vapor-phase nutrients to a contaminated subsurface region to stimulate the growth of contaminant-degrading microorganisms. The United States Government has rights in this invention pursuant to Contract No. DE-AC09-89SR18035 between the U.S. Department of Energy and Westinghouse Savannah River Company.
2. Discussion of Background:
Soil and groundwater contamination are ranked among the most serious pollution problems of the industrialized nations. It is estimated that over 15% of community drinking water supplies in the United States are contaminated with chlorinated hydrocarbons. Contamination damages the local ecosystem and may pose health problems if local groundwater is used as a source of drinking water or irrigation water, or if the soil is used for growing crops.
A number of methods are available for treating contaminated soil and groundwater. Soil can be treated simply by excavation and off-site disposal. Groundwater may be removed along with excavated soil or be pumped to the surface of the earth for treatment. In situ remediation techniques include stripping of volatile contaminants, as in the horizontal well systems described by Corey, et al. in commonly-assigned U.S. Pat. Nos. 4,832,122 (In-Situ Remediation System and Method for Contaminated Groundwater) and 5,186,255 (Flow Monitoring and Control System For Injection Wells), the disclosures of which are incorporated herein by reference.
Bioremediation of contaminated sites relies on natural processes to break down, immobilize or detoxify the contaminated soil or water. Bioremediation techniques are effective for treating many organic contaminants, however, bioremediation may be inefficient and time-consuming when compared to other treatment methods. Many months of treatment may be needed before contamination is reduced to acceptable levels.
Remediation rates can be increased by increasing the supply of nutrients for the indigenous contaminant-degrading microorganisms, for example, by increasing the concentrations of oxygen, nitrogen and phosphorous at the site, and adding trace inorganic nutrients such as iron, magnesium and manganese. In addition, indigenous microbial populations may be supplemented with naturally-occurring or genetically altered exogenous microorganisms. When the contaminant concentration decreases to acceptable levels, the nutrient supply is terminated and the microbial population returns to background, pretreatment levels. For example, Jhaveri, et al. (U.S. Pat. No. 4,401,569) pump contaminated groundwater to the surface, add microorganisms and nutrients such as oxygen, nitrogen, and carbon dioxide, then return the mixture to the ground for recirculation through the contaminated area to leach out and biodegrade the contaminants.
Instead of pumping contaminated groundwater to an aboveground treatment center, water, oxygen, and nutrients such as phosphates, nitrates and alkali metals may be supplied in situ via injection wells. See Ely, et al. (U.S. Pat. No. 4,765,902), Raymond (U.S. Pat. No. 3,846,290), Norris, et al. (U.S. Pat. No. 4,849,360). Bacterial cultures can be added to the site, together with air and nutrients, thereby facilitating metabolization of hydrocarbons in the soil (Hater, et al., U.S. Pat. No. 4,850,745).
Treatment may include the methods disclosed in commonly assigned patent applications Ser. No. 07/935,950, filed Aug. 27, 1992 (Bioremediation of Contaminated Groundwater) and Ser. No. 07/896,762, filed Jun. 10, 1992 (Method and System for Enhancing Microbial Motility), the disclosures of which are incorporated herein by reference. In the former application (Ser. No. 07/935,950), nutrients are injected cyclically to stimulate the growth and reproduction of indigenous microorganisms that are capable of aerobically degrading the contaminants. Treatment is carried out by periodically injecting a mixture of the nutrient and an oxygenated fluid, followed by injection of the oxygenated fluid alone. The nutrient builds up the concentration of microbes capable of degrading the contaminants; the absence of the nutrient forces the microbes to degrade the contaminants in an oxygen-enriched, aerobic environment for that degradation. Treatment is continued until the subsurface concentration of contaminants is reduced to a preselected level. The latter application (Ser. No. 07/896,762) discloses attracting indigenous microbes to the site by placing a quantity of tetrachloroethylene (TCE) near the contaminants.
The effectiveness of these techniques is limited by the ability to effectively disperse a sufficient supply of nutrients throughout the contaminated site. Injection of liquid nutrients (including liquids, liquid droplets and aerosols) has not been successful because liquids tend to adsorb to the soil near the injection location, thus do not disperse to provide general stimulation in the entire contaminated site. The resulting high concentration of nutrients near the injection point can lead to biological growth so immediate and rapid that it plugs the injection well, precluding further injection.
Nutrients that are supplied in gaseous form diffuse much more readily throughout the site, and thus are available to microorganisms at the site. Many nutrients, including oxygen, nitrogen, carbon dioxide, ammonia and methane, are widely available in gaseous form. However, there are no nontoxic compounds of phosphorus, a necessary nutrient for growth and reproduction, that are gaseous at environmental temperatures. Therefore, phosphorus is usually supplied in aqueous solution (Lawes et al., U.S. Pat. No. 4,749,491; Raymond, U.S. Pat. No. 3,846,290) or as an air-vapor mixture (Graves, et al., U.S. Pat. No. 5,178,491). Graves, et al. contact a phosphoric acid solution with a carrier gas (such as air) so that the gas picks up some of the phosphate molecules, then inject the mixture into a bioremediation site. Phosphoric acid (H.sub.3 PO.sub.4) is formed by dissolving solid phosphorus pentoxide (P.sub.2 O.sub.5) in water, thus, the phosphate concentration of the gas-vapor mixture is limited by the vapor pressure of the dissolved solid. Only a small fraction of the phosphorus in the solution is available for vapor-phase transport; most of the phosphorus is in the form of dissociated phosphate ions, which remain in solution and cannot evaporate into the gas. Consequently, large volumes of phosphoric acid are needed for sufficient nutrition. In addition, phosphoric acid is corrosive and difficult to handle, and the system does not allow precise control of the amount of phosphate transferred to the gas.
There is a need for an efficient, cost-effective method for delivering vapor-phase nutrients, particularly phosphates, to contaminated soil and groundwater to enhance in situ bioremediation. The ability to supply controlled amounts of phosphorous is critical to the future success and reliability of in situ bioremediation technologies.